xds_end2end_test: Fix race in security tests (#29408)
xds_end2end_test: Fix race in security tests (#29408)

* xds_end2end_test: Fix race in security tests

* Reviewer comments
jtattermusch/grpc,vjpai/grpc,vjpai/grpc,stanley-cheung/grpc,ctiller/grpc,ctiller/grpc,vjpai/grpc,grpc/grpc,grpc/grpc,vjpai/grpc,vjpai/grpc,nicolasnoble/grpc,ejona86/grpc,vjpai/grpc,grpc/grpc,nicolasnoble/grpc,ctiller/grpc,nicolasnoble/grpc,nicolasnoble/grpc,grpc/grpc,ctiller/grpc,ejona86/grpc,jtattermusch/grpc,vjpai/grpc,nicolasnoble/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,vjpai/grpc,vjpai/grpc,vjpai/grpc,jtattermusch/grpc,ctiller/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,grpc/grpc,grpc/grpc,ejona86/grpc,jtattermusch/grpc,grpc/grpc,ctiller/grpc,jtattermusch/grpc,nicolasnoble/grpc,ctiller/grpc,ejona86/grpc,ctiller/grpc,nicolasnoble/grpc,nicolasnoble/grpc,stanley-cheung/grpc,stanley-cheung/grpc,ctiller/grpc,nicolasnoble/grpc,jtattermusch/grpc,vjpai/grpc,stanley-cheung/grpc,ejona86/grpc,ctiller/grpc,jtattermusch/grpc,ejona86/grpc,ejona86/grpc,stanley-cheung/grpc,jtattermusch/grpc,jtattermusch/grpc,nicolasnoble/grpc,ejona86/grpc,nicolasnoble/grpc,grpc/grpc,ctiller/grpc,ejona86/grpc,grpc/grpc,ejona86/grpc,ejona86/grpc,stanley-cheung/grpc,vjpai/grpc,jtattermusch/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,nicolasnoble/grpc,jtattermusch/grpc